Note the connect(13, ...) and close(13) right after the response has been received.
For PHP it's working, but AFAIK there's nothing in the FastCGI protocol that the backend has to do other then keeping the connection open.

gettimeofday({tv_sec=1498125120, tv_usec=540583}, NULL) = 0
recvfrom(3, "GET /v2/rides HTTP/1.1\r\nHost: de"..., 1024, 0, NULL, NULL) = 561
socket(AF_UNIX, SOCK_STREAM, 0) = 13
ioctl(13, FIONBIO, [1]) = 0
epoll_ctl(10, EPOLL_CTL_ADD, 13, {EPOLLIN|EPOLLOUT|EPOLLRDHUP|EPOLLET, {u32=370685264, u64=94674334790992}}) = 0
connect(13, {sa_family=AF_UNIX, sun_path="/tmp/backend.socket"}, 110) = 0
getsockopt(13, SOL_SOCKET, SO_ERROR, [0], [4]) = 0
writev(13, [{iov_base="\1\1\0\1\0\10\0\0\0\1\1\0\0\0\0\0\1\4\0\1\3\344\4\0\17\36SCRIPT"..., iov_len=1040}], 1) = 1040
recvfrom(3, 0x7ffeb4895e57, 1, MSG_PEEK, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
epoll_wait(10, [{EPOLLOUT, {u32=370685264, u64=94674334790992}}], 512, 60000) = 1
gettimeofday({tv_sec=1498125120, tv_usec=546097}, NULL) = 0
epoll_wait(10, [{EPOLLIN|EPOLLOUT, {u32=370685264, u64=94674334790992}}], 512, 59994) = 1
gettimeofday({tv_sec=1498125120, tv_usec=550400}, NULL) = 0
recvfrom(13, "\1\6\0\1\7[\0\0Content-Type: applicatio"..., 4096, 0, NULL, NULL) = 1915
readv(13, [{iov_base=0x561b1610558b, iov_len=2181}], 1) = -1 EAGAIN (Resource temporarily unavailable)
writev(3, [{iov_base="HTTP/1.1 200 OK\r\nServer: nginx/1"..., iov_len=201}, {iov_base="712\r\n", iov_len=5}, {iov_base="\37\213\10\0\0\0\0\0\0\3\335\335Oo\334T\30\305\341}?E\325u\211\356?\277\347\274|\3"..., iov_len=1810}, {iov_base="\r\n", iov_len=2}], 4) = 2018
close(13) = 0
writev(3, [{iov_base="0\r\n\r\n", iov_len=5}], 1) = 5
write(4, "84.207.216.34 - - [22/Jun/2017:0"..., 203) = 203
recvfrom(3, 0x561b1616a130, 1024, 0, NULL, NULL) = -1 EAGAIN (Resource temporarily unavailable)
epoll_wait(10, [{EPOLLIN|EPOLLOUT, {u32=370684801, u64=94674334790529}}], 512, 65000) = 1